CS IT. Lecture (06) STP (I) Problem statement. By: Dr. Ahmed ElShafee

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1 Lecture (06) STP (I) By: Dr. Ahmed ElShafee CS IT Problem statement If your network consists of layer 2 switches that allow computers connect and exchange data, you will need to consider the design that can withstand some types of failure: Redundant Connections ١ Dr. Ahmed ElShafee, ACU : Spring 2016, Practical App. Networks I ٢ Imagine that the SW1, SW2 and SW3 switches connect many devices and there is only a single connection between the switches links between the switches break, the communication between many devices fail. Such design creates a single point of failure. We could easily enhance this simple design to make it more resilient by adding an extra path between SW2 and SW3 ٣ ٤

2 creating the extra path here comes at a cost. The redundant connection between SW2 and SW3 creates a loop. The loop in turn, will create three serious problems. The last one in the list will eventually render our system unavailable. 1. Duplicate Frame Delivery ٥ ٦ SW2 and SW3 do not have the MAC address of PC3 ( ) in their databases This can happen if the PC3 doesn't speak for more than five minutes (This is the default time MAC address is kept in the database without refreshing it) Then, we have PC1 sending frame towards PC3. As you recall, SW2 will flood the frame out of its active ports if it does not know where PC3 is located (unknown destination MAC address). The frame travels out SW2's port F0/13 towards SW1 and out the port F0/12 towards SW3. ٧ ٨

3 SW2 will deliver the frame to PC3. Since SW3 floods the frame out as well, it will be sent towards SW1 out of its port F0/14. Then, SW1 obediently delivers the same copy of the frame to PC3 again 2. MAC Address Table Instability switches change the MAC addresses depending on where they hear the sender. ٩ ١٠ PC1 sends the frame to PC3 (destination MAC: ). SW2 floods the frame out F0/12 and F0/13 ports. SW3 receives this frame sourced with MAC address (PC1). It learns the source MAC address and maps it to its F0/12 port where it arrived. SW1 does not know where PC3 is connected (at least right now) it will flood this frame out all active ports. This way, the frame is sent out SW1's port F0/14 towards SW3. SW3, upon receiving the frame on its F0/14 port, reads the source MAC address ( ) and maps it to port F0/14 this time. ١١ ١٢

4 This causes a little confusion as SW3 learned it earlier on and it was port F0/12 before. Previous mapping is removed and F0/14 becomes the outbound port for now. 3. Broadcast Storm problem is really severe. It can bring our traffic to a halt ١٣ ١٤ Spanning Tree Protocol Overview PC1 sends a broadcast frame. SW2 upon receiving it, floods it out all its active ports. SW1 receives it on port F0/13 and floods it out of other ports. SW3 receives the broadcast frame on its F0/12 port and floods it. it receives this same broadcast frame from SW1 and again it floods it out all active ports except the port it arrived on STP is a layer 2 loop prevention mechanism. Switches running this protocol use special frames called Bridge Protocol Data Unit (BPDU). These frames contain enough information to allow the switches to create a loop free topology. This is accomplished using three distinct phases: ١٥ ١٦

5 1. Elect a single switch to be the root bridge machine which is the central device in the layer 2 network. This machine will have all its ports in the forwarding state (designated port role). 2. All other switches (non root switches), will select a single path towards the root bridge. That port is called the 'root port' and will be forwarding traffic that is destined out of the switch through the root bridge. This path is the least cost (best) path towards the root. 3. All other switches will select a single path per segment in order to block stop the loop. The port that is forwarding traffic is called designated port. The port that is blocking traffic to stop the loop is called non designated port. ١٧ SW1 has been elected as the root bridge. SW2 uses port F0/13 as its root port (the best, or the least cost path towards the root). ١٨ SW3 uses it port F0/14 as the root port. SW3 blocks the port F0/12 to stop the loop. SW2 keeps sending BPDU frames originated by the root bridge (SW1) out its F0/12 port towards SW3. Spanning Tree Protocol Terminology The ports participating in STP play different roles and those roles use different states of operation: Spanning Tree Port Roles Root Port (RP) It is a port on a non root switch, which is the shortest (the best) path towards the root bridge. Root bridge does NOT have any root ports. (no shortest path to itself. ١٩ ٢٠

6 Designated Port (DP) It is a port that is in the forwarding state. All ports of the root bridge are designated ports (they are never in a blocking state). BPDU frames our sent out this port. Non Designated Port (NDP) It is a port that is in a blocking state in the STP topology. ٢١ ٢٢ Spanning Tree Port States Disabled The port in this state does not participate in the STP operation (it is shut down). Blocking The port does NOT forward any Ethernet frames, does NOT accept any Ethernet frames (discards arriving frames), does NOT learn any MAC addresses. However, the port DOES process BPDU frames received from a neighboring switch. If the port transitions to this state (blocking), it can stay blocked for 20 seconds by default (max_age). Listening The port in this state CAN send and receive the BPDU frames. However, the port in this state does NOT learn any MAC addresses, and does NOT forward or process incoming frames either. All Ethernet frames are being discarded. The computation of loop free topology takes place in this state. If the port transitions to this state (listening), it can stay in this state for 15 seconds by default (forward_delay). ٢٣ ٢٤

7 Learning The port in this state already knows its role (root port or designated port ) in the STP domain. However, the port will not forward any Ethernet frames yet. It will be learning MAC addresses from the frames arriving at the port in order to populate MAC address table. This helps avoid too much flooding when the port transition to the forwarding state. If the port transitions to this state (learning), it can stay in this state for 15 seconds by default (forward_delay). Forwarding The port in this state will forward all Ethernet frames as per switch operation. Also, the port will process all incoming Ethernet frames and will actively learn MAC addresses from the arriving traffic. ٢٥ ٢٦ Step one, root bridge Root Bridge Root bridge is the switch that has all ports working in the designated role. It will be the reference point Root bridge will impose the timers that other switches will use such as: hello time how often BPDUs are going to be sent/relayed (default timer=2 seconds), max age how long the configuration is valid (default timer=20 seconds), forward delay how long a port should be in ٢٧ listening/learning state (default timer=15 seconds). Root bridge will be announcing its presence by sending BPDU frames. Other switches will relay those frames out their designated port given the hello time. Also, the root bridge has all its ports in the designated role (forwarding). ٢٨

8 Root election is based on a single parameter that is found in the BPDU frame called: Bridge ID. The switch with the lowest Bridge ID becomes the root. Bridge ID has the following format: priority.base mac address Priority is configurable parameter that is used to elect the root bridge a device you want to be the root. The default value is: The lower the value is the more likely for a switch to become a root. Base Mac Address is the unique mac address every switch has been given by the manufacturer. It is a main parameter in case the priority on all switches is ٢٩ identical. ٣٠ start up all the switches and as soon as their ports transition to LISTENING state, they begin to send BPDU frames out of all active ports. In those frames both Bridge ID and Root ID parameters point to their own priority.base mac address value. Since they are processing the incoming BPDU's from the neighbors, SW2 and SW3 realize that SW1's Bridge ID is lower than theirs. From that point onwards, they begin to relay BPDU frames saying that SW1 as the root bridge. ٣١ ٣٢

9 example, For SW3 Bridge ID = Root ID = Step 2: Root Port Selection As soon as the root has been elected, all non root switches begin to calculate which port is the best (the least cost) towards the root bridge. This port will be called the root port. ٣٣ ٣٤ SW2, SW3 and SW4 receive BPDUs from different directions. For instance, SW2 will receive them on its port F0/1 and F0/2 The accumulative cost (the sum of the cost in the path towards the root), is taken into consideration. The lowest cost to reach the root becomes the root port. cost of path calculation: Each speed has its arbitrarily assigned cost which is configurable. A few examples are below: 10 Mbps = Mbps = 19 1 Gbps = 4 10 Gbps = 2 ٣٥ ٣٦

10 The root bridge (here SW1) is sending its BPDU frame every 2 seconds. It uses the parameter called: Root Path Cost in BPDU to advertise the cost to the root. It puts the value of '0' in it, as it is the root bridge and has no cost to itself. The frame is sent out its port F0/1 towards SW3 and F0/2 towards SW2. SW2, upon receiving it, adds the cost used to reach the sender of BPDU based on the predefined speed to cost value (all ports in our topology are ٣٧ FastEthernet=19). Root Path Cost = = 19 via F0/2 SW2 is going to advertise its best (as of now) cost out of F0/1 port towards SW3. SW3 will receive BPDU from SW1 with the Root Path Cost=0 on its F0/1 port. It will also receive BPDU from SW2 on its F0/2 interface with the Root Path Cost=19. SW3; As both ports have the cost of 19 towards those BPDU senders, the following math is done to choose the least cost path towards the root bridge Root Path Cost = = 19 via F0/1 ٣٨ Root Path Cost = = 38 via F0/2 It is clear that the direct connection towards root bridge via F0/1 is going to be selected as the root port. SW3 has the least cost towards equal 19 (via F0/1 port). This cost is going to be added to Root Path Cost while it sends the BPDUs out F0/2, F0/3 and F0/4. Of course, SW2 also chooses its F0/2 port as the root port since the cost is smaller. if the Root Cost Path is identical! hat situation on SW4. It receives BPDUs on its ports F0/1 and F0/2 with the following parameters: Bridge ID = Root ID = Root Path Cost = 19 ٣٩ ٤٠

11 refer the lowest port priority of the sender. That parameter has a default value 128 and is configurable. The designated switch (SW3), is the same switch i.e. the same Bridge ID ( ). The designated switch (SW3) sends BPDUs out of its F0/3 and F0/4 ports with the same priority = 128 lowest Port ID where BPDU frames arrive on SW4. Port f0/3 becomes the root port since F0/3 is lower than F0/4 on SW3. ٤١ ٤٢ The following algorithm is used to determine the root port or designated port (in order): 1. Prefer the lowest Root Path Cost. 2. In case of the same Root Path Cost, prefer the lowest Bridge ID of the designated switch (the neighbor that sends BPDUs). 3. In case of receiving BPDUs on multiple ports from the same designated switch (BPDU sender), prefer the lowest port priority of the sender. That parameter has a default value 128 and is configurable. 4. In case of all above are did not resolve the problem, prefer the lowest Port ID of the BPDU sender. Step 3: Designated Port Selection. This procedure follows exactly the same algorithm used for root port selection. ٤٣ ٤٤

12 Since root port is the best port towards the root bridge it is going to be in the forwarding state Root Path Cost advertised by SW2 is 19 and so is the cost advertised by SW3. SW2 has lower Bridge ID ( ) than SW3 ( ). SW3 must block its F0/2.. And last selection is going to happen between SW3 (port F0/4) and SW4 (port F0/2). Root Path Cost Advertised by SW3 is 19, but SW4 advertises its cost as 38 (two hops via F0/1). SW4 blocks its port F0/2 (nondesignated), the SW3 promotes its port F0/4 to designated role (forwarding). ٤٥ ٤٦ This process happens in the LISTENING state of all ports. Since the topology has been computed and does not have loops (blocking appropriate ports), it is safe to move to next states: learning and finally forwarding. Thanks,.. See you next week (ISA), ٤٧ ٤٨ Dr. Ahmed ElShafee, ACU : Spring 2016, Practical App. Networks I